The present invention relates to apparatus for measuring the beam current of beams of charged particles, particularly ion beams or electron beams. Reliable ion beam current measurements and electron beam current measurements are respectively necessary in ion implantation systems and in electron beam bombardment systems since the implanted dose or the extent of the electron bombardment is directly related to the beam current. Beam current measurements are complicated by the fact that energetic ions or electrons impinging on a solid surface eject secondary electrons and ions. Any surface such as the beam-defining apertures or the implantation target and its mounting hardware can be sources of such secondary electrons or ions. Due to the presence of such secondary particles, conventional measuring systems will not provide an accurate representation of the true beam current since any secondary particles arriving at or leaving the target will affect the target current.
Monitoring of ion implantation currents or dosages with Faraday Cages is known in the art. The text, "Ion Beams With Applications to Ion Implantation", R. G. Wilson and G. R. Brewer (1973, pp. 446 - 449, 451 - 452, and 457) discusses the conventional utilization of the Faraday Cage for ion beam current or dosage monitoring. In such monitoring, the cage may be a separate unit retractably inserted between the target and the beam for occasional monitoring, or the Faraday Cage may consist of the actual target in combination with the housing walls for continuous monitoring during the ion implantation operation. In either case, the actual target or the target plate (in the case of the retractable cage) is fully insulated from the cage walls. In such conventional Faraday Cage monitoring in the case of the primary ion beams which are always positively charged, the target is conventionally maintained at a potential which is positive with respect to the cage walls maintained at a negative voltage level. In such a structure, the cage walls purportedly function to suppress secondary electrons created at the target back toward the target so that the loss of such electrons will not affect the target current which is the only current measured by an ammeter and similar measuring apparatus in determining current or dosage. However, we have found that such conventional Faraday Cages appear to not take into account the secondary ions which are also created at the target. The negative potential of Faraday Cage walls will attract such ions and will thereby produce an adverse effect on the beam current measured only from the target.
Furthermore, the negative bias on the cage walls should cause some undesirable ion beam expansion beyond the desired limits of the beam or the limits of the beam target.